The present invention relates to a method for recovery of zinc from plating waste water solutions and the return of concentrated zinc solution to the plating bath. More particularly, the present invention relates to membrane-based solvent extraction processes.
In recent years, there has been an increasing need for the removal of metal ions from aqueous solutions not only for reasons of pollution control, but also for the purpose of recovering valuable mineral species. In particular, the recovery of zinc from the waste water solution from plating plants has been found to be particularly desirable for both of these reasons. Furthermore, recycling of zinc values contained within the waste water solution back to the plating bath would significantly increase the economics involved in zinc plating operations.
In the past, electrolytic reactors have been developed and employed for the removal and recovery of metals from plating plant waste water solutions. Although such processes are applicable to the recovery of many heavy metals, these processes are not generally effective for the treatment of acidic zinc solutions. This is mainly due to the low current efficiencies associated with a competing reaction (that is, hydrogen evolution) occurring at the cathode. Recycling metals using these electrolytic processes requires disassembly of the cell to reuse the metal deposits or dissolution of the metal with an acid. In contrast, the process disclosed herein produces a solution concentrated with zinc and recycles it directly to the plating bath solution without the requirement of any further treatment.
Other processes for the separation of mineral values from various solutions include solvent extraction methods. The separation of molybdenum from tungsten-containing solutions is described in the article "The Recovery of Molybdenum From Spent Mandrel Acid by Solvent Extraction" by D. S. Flett, J. Mellig and E. W. West on pages 80-149 of the Proceedings of the International Solvent Extraction Conference, held Sept. 6-12, 1980 in Liege, Belgium. Another solvent extraction process for metal separation is disclosed in U.S. Pat. No. 3,969,478, issued July 13, 1976 to Zelikman et al. Although such methods are capable of separation and/or the recycling of metals, these processes have generally exhibited one or more problems. For example, a significant problem is the loss of solvent during the process. Another problem that occurs is the difficulty encountered in the separation of organic and aqueous phases in the resulting solutions. Additionally, these processes generally require multi-stage operation to achieve a high enrichment factor. Moreover, they can be particularly susceptible to particulate contamination.
In addition to the above electrolytic and solvent extraction methods for metal separation, various membrane-based methods have been employed in the separation of metal values. In particular, in U.S. Pat. No. 4,306,946, issued Dec. 22, 1981 in the name of the instant inventor, Donnan dialysis is employed as a continuous ion exchange process. This process employs a solid ion exchange membrane as a barrier between feed and eluant solutions. However, the method suffers from low removal rate, which is determined by the diffusion rate of ions through the solid, ion exchange membrane. However, it is nonetheless a continuous method and provides many advantages over conventional solvent extraction methods employing mixer/settler systems. Another form of membrane-based metal recovery process is described in the article "Coupled Transport Membranes" by R. W. Baker et al. in the Journal of Membrane Science, Vol. 2, pages 213-233, 1977. The process described therein employs a porous membrane which is impregnated with an organic absorbing medium. However, this process exhibits certain short-comings, particularly membrane instability. Membrane extraction performance deteriorates significantly due to the loss of the organic medium and also due to the filling of the pores with the aqueous solution. This happens since the organic medium is leached out of the pores or pushed out of the pores due to the pressure imbalance across the membrane. The present invention is distinct from the method described therein in many respects, most notably in that the liquid ion exchange material is not impregnated in the membrane but rather is disposed in a recirculating system between separate membranes in distinct fluid-tight chambers. Accordingly, the problem of the loss of ion exchange material is not present.
Additionally, a process for hydrometallurgical extraction is described in U.S. Pat. No. 3,957,504, issued May 18, 1976 to Ho et al. FIG. 4 of the patent to Ho et al. is particularly relevant in that it illustrates a hydrometallurgical extraction process flow diagram in which a chelating fluid is recycled back to a first ionic membrane extraction unit. However, no provision is made in the apparatus for treatment of the recycled chelating solution. In particular, there is no provision for the use of an aqueous/organic separator which the present inventor has found to be particularly effective in selectively isolating and enriching desired metallic species using porous membranes.